Shijun Han

The chemical nature of the C–H⊕⋯X− (X=Cl or Br) interaction in imidazolium halide ionic liquids

The C–H⋯X (X=Cl or Br) interaction is traditionally characterized as a relatively weak interaction. However, this interaction becomes very strong in the imidazolium-based halide ionic liquids [J. Phys. Chem. 123, 174501 (2005)]. This strong interaction had been attributed to the electrostatic interaction between the imidazolium cation and the halide anion. In this paper, the chemical nature of the C–H⊕⋯Cl− and C–H⊕⋯Br− interactions is investigated by atoms in molecules (AIM) and natural bond orbital (NBO) analyses. The AIM calculations indicate that in the EmimX complexes, the C–H⊕⋯Cl− and C–H⊕⋯Br− interactions have some covalent character, especially the C–H⊕⋯Cl− interaction. Mulliken, ChelpG charge, and natural bond orbital population analyses for these two kinds of interactions indicate that the charge transfer is important in the interaction of the cation with the anion. In addition, the NBO analysis demonstrated that the stabilization energy is due to an n→σC–H• orbital interaction. However, in the E...

Structure and conformation properties of 1-alkyl-3-methylimidazolium halide ionic liquids : A density-functional theory study

The structures and conformational properties of 1-alkyl-3-methylimidazolium halide ionic liquids have been studied with a Beckes 3 Parameter functional method. The interaction mechanisms between the cation and the anion in 1-ethyl-3-methylimidazolium (Emim+) halide and 1-butyl-3-methylimidazolium (Bmim+) halide ionic liquids were investigated using 6-31G*, 6-31++G**, and 6-311++G** basis sets. Forty structures of different ion pairs were optimized and geometrical parameters of them have been discussed in details. Halide ions (Cl- or Br-) have been gradually placed in different regions around imidazolium cation and the interaction energies between the anion and the cation have been calculated. Theoretical results indicate that there are four activity regions in the vicinity of the imidazolium cations, in these regions the imidazolium cations and the halide anions formed stable ion pairs. Imidazolium cations can form hydrogen bond interactions with one, two or three but no more than three nearest halide anions. The halide ions are situated in hydrogen bond positions rather than at random.

The methyl C-H blueshift in N,N-dimethylformamide -water mixtures probed by two-dimensional Fourier-transform infrared spectroscopy

Two-dimensional correlation spectroscopy was used to study the composition-dependent spectral variations of the CH-stretching bands of N,N-dimethylformamide (DMF)-water mixtures with X(DMF) ranging from 0.98 to 0.60. By a detailed correlation analysis of the spectral changes of the CH- and OH-stretching bands, it is found that the intensities of the CH and OH bands change in different ways when the water content is increased. It is also found that two different regions of the water content can be distinguished, in which the intensity changes have different signatures. A tentative explanation for how these phenomena might be related to structural changes in the mixture is proposed. The structural change of DMF induced by the water hydrogen bonded on the carbonyl group is supposed to be the possible origin of the methyl C-H blueshift instead of the direct C-H...O interactions before the hydrophobic hydration takes place.

NMR and Excess Volumes Studies in DMF–Alcohol Mixtures

Chemical shifts of the alcohol and DMF protons in DMF–alcohol mixtures with the mole fraction of alcohol are reported in order to study the hydrogen bond interaction present in the mixtures. The densities of DMF–methanol mixture at 22°C are also measured. Excess volumes and excess chemical shifts are correlated by the Redlich–Kister equation. The relation between excess volumes and excess chemical shifts in the mixtures is discussed. It is found that the maximum excess chemical shifts δE(CHO-OH) and δE(CH3-OH) are positioned at about mole fraction methanol = 0.57 for the DMF–methanol system, as is VE. The results show that the NMR spectral method offers a valuable approach to similar future studies of interactions in mixtures.

Simple local composition model for 1H NMR chemical shift of mixtures

Abstract Various chemical association models have been developed to fit the chemical shift data of mixtures. However, pure chemical models retain the primary disadvantage of multiple adjustable parameters. A simple semi-empirical physical local composition model is presented in this Letter. The 50 sets of data for 14 systems were correlated, indicating that the new model can well correlate the 1 H NMR chemical shift data of binary mixtures with only one parameter. The distinguishing feature of this model is that the built-in temperature dependence of parameter makes it able to predict the temperature variation on NMR chemical shift data.

Vapor–liquid equilibrium data for methanol–water–NaCl at 45°C

Abstract Isothermal vapor–liquid equilibrium data at 45°C for methanol–water–NaCl system have been measured by using the two-phase-recirculating still. The compositions were determined by densities. The data were tested by the thermodynamic consistency, which was based on reduced excess free enthalpy function.

(Vapour + liquid) equilibria for the binary mixtures (cis-pinane + α-pinene) and (cis-pinane + 1-butanol)

The isothermal and isobaric (vapour + liquid) equilibria for (cis-pinane + α-pinene) and (cis-pinane + 1-butanol) measured with an inclined ebulliometer are presented. The experimental results are analysed using the UNIQUAC equation with the temperature-dependence binary parameters with satisfactory results. Experimental vapour pressures of cis-pinane are also included.

Prediction among different spectroscopic properties for aqueous systems.

Spectroscopic properties obtained by NMR and vibrational spectra both reflect the microscopic environment of solutions, and the local composition (LC) theory can be used to study environmental effects on spectroscopic properties. Based on the LC model, the relationship between NMR and vibrational spectra, including infrared (IR) spectroscopy and Raman, were investigated. For the aqueous systems-water+N, N-dimethylformamide, water+acetone, water+methanol, and water+ethanol, we performed prediction between concentration-dependent peak positions of IR and Raman, as well as between concentration-dependent vibrational properties and 1H NMR chemical shifts. The results showed that reliable prediction could be achieved with the help of the LC model. This suggests that 1H NMR chemical shifts and vibrational spectroscopic properties may tell us the same story about the local environment encountered in solution.

Thermodynamic Properties of the Ternary System Potassium Bromide + Lithium Bromide + Water at 25°C

Activity coefficients of potassium bromide in aqueous mixtures of potassium bromide and lithium bromide were determined by emf measurement at 25°C and at six ionic strengths from 0.1 to 2.5 mol·kg-1. The experimental data were fitted using the Scatchard–Rush–Johnson and Pitzer models. The osmotic coefficients, excess Gibbs energies of mixing, and activity coefficients of lithium bromide in aqueous mixtures were calculated using Pitzer mixing parameters obtained in this work.

Exploring a new kind of aromatic hydrogen bond: hydrogen bonding to all-metal aromatic species

Based on the recent advance of the aromaticity concept into all-metal species, we put forward a new kind of hydrogen bond: hydrogen bonding to all-metal aromatic species. The similarities and differences between the hydrogen bonds in all-metal aromatic systems and those in traditional organic aromatic systems are systematically explored: we investigated the interactions between all-metal aromatic complexes Al42−/Al4M (M = Mg2+, Ca2+, 2Na+) and H–Y (Y = CH3, NH2, OH, F, Cl, Br) as well as those between benzene and H–Y. Geometric configuration calculations, molecular orbital (MO) analysis, nuclear independent chemical shift (NICS) and frequency shift (FS) as well as binding energy calculations were employed for the investigation of the Al42−⋯H–Y, Al4M⋯H–Y and C6H6⋯H–Y complexes. The calculated results revealed that there exist distinct differences between the hydrogen bonds in all-metal aromatic systems and those in traditional organic aromatic systems in many aspects such as binding energy and interaction distance as well as frequency shift of H–Y. Furthermore, because the aromaticity of all-metal aromatic systems can be changed by interaction with different metal ions, the intensity of hydrogen bonding can also be adjusted and controlled by changing the ions. The natures of hydrogen bonds in all-metal aromatic systems are summarized.